Galileo Galilei was born in Pisa,
Italy, on February 15, 1564. He showed unusual skill in
building toys as a child, played the lute and the organ
well, and won a reputation for his excellent paintings.
His father, a merchant and musician, taught Galileo
music, but encouraged him to become a doctor. Galileo
studied medicine and the philosophy of Aristotle at the
University of Pisa.

Young Scientist

Galileo made his first important
scientific discovery while he was still a university
student. While at a service in Pisa Cathedral, his
attention was caught by the swaying of the chandeliers
above his head. Struck by their steady rhythm, he
designed a simple pendulum in order to investigate the
regularity of its swing. Using his own pulse beats to
measure the swing, he discovered that each swing took the
same amount of time regardless of the length of the arc.
He then suggested that pendulums might therefore be used
to regulate the measurement of time, an idea that was
later applied by Christiaan
Huygens.

left: a model constructed from
Galileo's design for a pendulum clock
right: copy of Galileo's design for an escapement or
pendulum clock

Galileo left the university in 1585 due
to lack of funds, and abandoned medicine for research in
mathematics. During this time, he invented the
hydrostatic balance, which is used to find the specific
gravity of objects by weighing them in water.

Galileo returned to the University of
Pisa at the age of 25 as professor of mathematics. During
this period, he is credited with discovering the law of
falling bodies. Reasoning that gravity pulls all bodies
to earth with the same acceleration, regardless of their
weight, Galileo is said to have dropped two unequal
weights from the top of the Leaning Tower of Pisa
to prove his theory. According to a much-told story, a
crowd of students, professors, and priests looked on as
both weights struck the ground at about the same time.
Whether the story is entirely accurate or not, there is
no doubt that Galileo did openly dispute the long-held
theory, originally proposed by Aristotle, that heavy
bodies fall faster than lighter ones, and that Galileo
was forced to leave the university because of his views.

Astronomical Discoveries

In 1592, Galileo became professor of
mathematics at the University of Padua, where he remained
for 18 years. In 1597, he invented the sector, a
type of compass still used by draftsmen. Beginning in
1609, he built many telescopes and sold them throughout
Europe. He also made larger and more powerful telescopes
than had been made before.

Galileo's first important astronomical
observations were of the Moon, and he once again found
himself opposing the teachings of Aristotle. He
discovered that the moon was not a smooth sphere shining
by its own light, but that its surface was actually
marked by valleys and mountains and that it showed only
the light it reflected. He also studied the Milky Way and
found that our galaxy is a mass of stars "so
numerous as to be almost beyond belief."

one of Galileo's views of the Moon

In 1610, Galileo discovered the four
brightest satellites of Jupiter. He named these the Medicean stars, after the
Medici family, who ruled the province of Tuscany, where
he was born. That same year, he observed the peculiar
form of Saturn, the rings of
which would later be recognized by Christiaan Huygens.

These discoveries added support to the
theory put forward by Nicolas Copernicus,
that the Earth moves around the Sun. But they also
brought him extreme abuse. Many churchmen and follwers of
Aristotle opposed Galileo. But Cosimo II, a member of the
Medici family and Grand Duke of Tuscany, became his
patron and invited Galileo to serve as his personal
mathematician in Florence and the University of Pisa.

In Florence, Galileo detected the
phases of Venus and a slight
phase of Mars. In Rome, he used
one of his telescopes to show Pope Paul V and other high
church officials what he had discovered. In spite of
these demonstrations, a dispute followed between
churchmen and scientists. The Church also bitterly
opposed Galileo's report on sunspots.

In the 1620's, Galileo published a
paper outlining the basic ideas of what is today known as
the scientific method. He proposed that the results of
experiments should form the basis of mathematical
formulations of new theories, and that these theories
should themselves be tested by further experiment. He
also argued that the tradition of treating mathematics
and science as separate disciplines should be discarded.

The Inquisition

In 1632, Galileo published his
masterpiece, A Dialogue on the Two Principal Systems
of the World. The Holy Office, or Inquisition,
immediately called him to appear before it. After a long
trial, Church officials forced him to say that he no
longer believed in the Copernican theory, and sentenced
him to an indefinite prison term. Instead of imprisoning
him, however, they confined him to his villa in Florence.

Galileo spent his last years writing on
the laws of force and motion. Dialogues on the Two
New Sciences, published in 1638, summed up his
life's work on motion, acceleration, and gravity, and
furnished a basis for the three laws of motion laid down
by Sir Isaac Newton in
1687.

Galileo died on January 8, 1642, and
was buried in the Church of Santa Croce in Florence.
Fifty years after his death, the city erected a monument
at the church in his honor.

In 1992, Pope John Paul II
formally proclaimed that the Roman Catholic Church had
erred in condemning Galileo.